fii daily trade data for derivatives

[varshar_100]

dear members,

i want to know something about daily fii derivative trade data provided by nse.

they provide data in eod file named fii_stats_date.xls.

they provide index future and option OI contracts
but many times you can see that OI data dont seems to be correct.

like :
date buy sale oi

10-8 100 90 10
11-8 100 80 22 oi should be 30 as buy and sale differance is 20.

please help me understanding these data.

thanks